The invention relates generally to circuits, and more particularly to a circuit and method for correcting thermal deviations of an output signal from an amplifier.
Signal dependent changes in the temperature of xe2x80x9csignalxe2x80x9d transistors in an amplifier can cause gain changes or thermal deviations, which are reflected in the output signal of the amplifier over time in the form of a xe2x80x9cthermal tailxe2x80x9d. xe2x80x9cSignalxe2x80x9d transistors are transistors on the signal path of the amplifier. A thermal tail is part of an amplified signal that deviates from a defined level over time due to the temperature changes of the amplifier signal transistors. Consequently, in response to an input step signal, an amplifier may produce an output signal that seems to settle within a few hundred picoseconds but instead continues to slowly change toward a final value over several milliseconds. This is particularly problematic if the amplifier is used in oscilloscopes where step settling can easily be observed over many decades of sweeps. Typically, a thermal tail correcting circuit is used to correct thermal deviations of an output signal from an amplifier.
A conventional thermal tail correcting circuit operates to generate an offset signal, which corresponds to the gain changes due to temperature changes of signal transistors in an amplifier. The offset signal is generated using one or more active devices that are similar to the signal transistors in the amplifier. Using the output signal of the amplifier, the active devices of the thermal tail correcting circuit are operated under similar conditions as the signal transistors in the amplifier to correspondingly change the temperatures of the active devices. In the case in which the active devices are bipolar transistors, changes in the power dissipation of the active devices will lead to temperature changes, which arc reflected in the base-emitter voltages. Thus, the base-emitter voltages can be used to generate the offset signal.
A concern with a conventional thermal tail correcting circuit that uses bipolar transistors as the active devices is the introduction of base-width variations known as xe2x80x9cEarly effectxe2x80x9d in the bipolar transistors, which also changes the base-emitter voltages. Thus, the offset signal generated by the thermal tail correcting circuit is corrupted by the Early effect in the active devices, which can lead to degradation in the step response of an amplifier.
In light of this concern, what is needed is a circuit and method for correcting thermal deviations of an output signal from an amplifier with Early effect compensation.
A circuit and method for correcting thermal deviations of one or more output signals from an amplifier utilizes Early effect-compensated correcting signals to reduce the thermal deviations of the output signals. The correcting signals are derived from temperature-dependent signals, which correspond to the thermal deviations of the output signal. The temperature-dependent signals, however, include errors due to Early effect. The Early effect errors are compensated in the correcting signals by introducing reverse Early effect errors into the correcting signals. Consequently, the compensating signals can be used to more effectively correct the thermal deviations of the output signals.
A circuit for correcting thermal deviations of an output signal in accordance with one embodiment of the invention includes a thermal sensing unit and an error compensating unit. The thermal sensing unit includes a first transistor that is connected between a first high voltage terminal and a first low voltage terminal to generate a temperature-dependent signal in response to a first input signal. The first temperature-dependent signal includes an error caused by a voltage change induced by an effective base width variation of the first transistor, e.g., Early effect errors, in responding to the first input signal. The error compensating unit includes a second transistor that is connected between a second high voltage terminal and a second low voltage terminal to generate a correcting signal using the temperature-dependent signal. The second transistor is further connected to be responsive to an inverse signal of the first input signal to introduce a reverse error, e.g., a reverse Early effect error, into the correcting signal to compensate the effects of the error on the correcting signal. The correcting signal corresponds to the thermal deviations of the output signal. Thus, the correcting signal can be used to reduce the thermal deviations of the output signal.
A method for correcting thermal deviations of an output signal in accordance with one embodiment of the invention includes receiving a first input signal and a reverse signal of the first input signal, generating a temperature-dependent signal in response to the first input signal using a first transistor, generating a correcting signal in response to the temperature-dependent signal using a second transistor, and reducing the thermal deviations of the output signal using the correcting signal. The temperature-dependent signal includes an error caused by a voltage change induced by an effective base width variation of the first transistor, e.g., an Early effect error, in responding to the first input signal. The generating of the correcting signal includes operating the second transistor to be responsive to the inverse signal to introduce a reverse error, e.g., a reverse Early effect error, into the correcting signal to compensate the effects of the error on the correcting signal.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.